Trace the outline of the cut-in box along the wall where you would like to put the outlet safe. Use a drywall saw to cut out the rectangle.
Assemble the face plate. Align the receptacle over the cut-in box and push the roofing nails through the aligned holes at the top and bottom. Screw the wall plate to the receptacle.
Remove the assembled face plate from the box and screw the box into the drywall.
Insert items into the box that you would like hidden.
Replace the face plate by aligning the roofing nails in the box in the wall.